confusion over acl/mahle piston specs

[Sam_Q]

On the acl perfomance website they list both a forged 16V and 20V piston and quote them to be both for a 8.5:1 compression. Thats all good and all if you want that but I dont think it works out. For a start here is the 16V piston:

http://www.aclperformance.com.au/Toy...gedPistons.htm

and a pic:



It quotes it as having a 17.7cc dish,

Working it out with a 8.5:1 ratio it would have a total of a 53cc chamber, so 53-39cc(head chamber) -5.9cc (1.15 head gasket) - 1.5cc (0.3mm piston to block spacing) works out to be 7.3cc of piston dish. So I would think this piston would be much lower than 8.5:1


Now the 20V piston...

http://www.aclperformance.com.au/Toy...gedPistons.htm



This piston however is quoted with a 3cc dish

Again with a 53cc chamber needed for 8.5:1 compression but this time with a smaller 35cc chamber (according to arias) it would work out to 53cc - 35cc (chamber volume) - 5.9cc (gasket volume) - 1.5cc (0.3mm piston to block spacing) = 10.6cc so I think in reality the compression would be much higher.

Even visually looking at that 20V piston I can tell its only a few CC off a stock piston so theres no way it would be a significantly lower compression.

Anyone got any ideas?

[Big T]

Modified pin height perhaps?

Eddie.

[jeffro ra28]

Pin height will not effect CR unless it alters the overall deck height.

could you be certain that "Piston Crown Capacity" is the same as piston dish?

Are you sure a 4age has 39cc chamber?

With your information, i get an obviously rough result of 7.26:1

My bet is that the "17.7 cc" dish volume is a typo and ment to be 7.7.
With a 7.7 inputed dish volume i get a CR close to 8.5.

[Sam_Q]

eddie: I thought of that but the compression height is the same so thats not it.

jeffro: could piston crown capcity be anything but the dished volume?

I got that 39cc figure from arias, cant confirm it for sure.

I think your right about the typo its the only way to explain it. However my main interest is in the 20V piston which is more inexplicable.

[jeffro ra28]

the only real way to overcome these issues is take the sample piston and check it with CC gauge yourself.

[oldeskewltoy]

Sam... here is a silvertop piston....



The slug you show does have less material on top then the silvertop slug.......

the silvertop slug has the center section raised.... over 3mm....

[Sam_Q]

yeah I have one about 20cm from my keyboard but thanks anyway. My point is that the difference between the stock one and the acl one is the loss of the small central domed part which I previously worked out to be roughly 4cc I think. Accoding to my calculations what they are offering will have around 10:1 compression.